A wide variety of IMDs that employ electronic circuitry for providing various therapies such as electrical stimulation of body tissue, monitoring a physiologic condition, and/or providing a substance are known in the art. For example, cardiac pacemakers and implantable cardioverter-defibrillators (ICDs) have been developed for maintaining a desired heart rate during episodes of bradycardia or for applying cardioversion or defibrillation therapies to the heart upon detection of serious arrhythmias. Other devices deliver drugs to the brain, muscle and organ tissues, and/or nerves for treatment of a variety of conditions.
Over the past 20 years, the IMDs have evolved from relatively bulky, crude, and short-lived devices to complex, long-lived, and miniaturized IMDs that are steadily being miniaturized with their functionality continuously increasing. For example, numerous improvements have been made in cardioversion/defibrillation leads and electrodes that have enabled the cardioversion/defibrillation energy to be precisely delivered about selected upper and lower heart chambers and thereby dramatically reducing the delivered shock energy required to cardiovert or defibrillate the heart chamber. Moreover, the high voltage output circuitry has been improved in many respects to provide monophasic, biphasic, or multi-phase cardioversion/defibrillation shock or pulse waveforms that are efficacious, sometimes with particular combinations of cardioversion/defibrillation electrodes, in lowering the required shock energy to cardiovert or defibrillate the heart.
The miniaturization of IMDs is driving size and cost reduction of all IMD components including the electronic circuitry components, where it is desirable to reduce the size so that the overall circuitry can be more compact. As the dimensions of the IMDs decreases, the electronic circuits of the IMD circuitry are preferred to decrease power consumption in order to maintain or increase longevity. Furthermore, as the dimensions of the components are also shrinking, it is desirable to reduce the number of components within the IMD package.
One response to this desire has been through technological improvements to the existing components. For example, IMDs generally include capacitive sensors that include multiple components. Such sensors include accelerometers, pressure transducers, and similar transducers that may employ a capacitance system to detect, for example, position or motion. In some implementations, the sensors may include a sensing element that includes two parallel plate capacitive components acting in a differential manner in which acceleration of the sensor causes one of the capacitive components to increase in capacitance and the other capacitive component to decrease in capacitance. At rest, or at a constant acceleration, the difference between capacitances in the sensor may remain constant. A detection circuit may determine the values of the capacitances in the sensor by applying a voltage to the capacitive components, e.g., a square wave voltage, and producing an output voltage associated with the capacitive components. The output voltage produced may be digitized using an analog to digital (A/D) converter in order to produce a digital value that indicates the amount and direction of acceleration that is suitable for use in digital systems. Typically, the detection circuit used for determining the digital values of the two capacitances may include amplifiers, filters, oscillators, A/D converters etc.
It is desirable to provide improved techniques and circuits for capacitive sensing that overcome the limitations of the conventional state of the art capacitive sensors.